The present invention relates to a method for purifying water, in particular ground water, under anaerobic conditions, using a membrane filtration unit. The present invention furthermore relates to a device for purifying water, in particular ground water, under anaerobic conditions, using a membrane filtration unit which is provided with the necessary valves, supply and discharge pipes and pumps, and to drinking water obtained by using such a method.
Netherlands patent application 82 00080, which is laid open to public inspection, relates to a method for obtaining water by using membrane filtration, whereby it is stated that the water to be filtrated must be prevented from coming into contact with air. If the water contains iron compounds, for example, said iron compounds will be converted into solid substances under the influence of the ambient air, which solid substances will then deposit on the membranes. In order to prevent contact with the ambient air, the mouth of the borehole to the aqueous liquid is sealed substantially gastight. In practice it has become apparent, however, that a considerable amount of flocks is formed when such a plant is started, which flocculation can be ascribed mainly to the presence of air in the supply pipes and to the membrane surface (which is still dry). Thus, said method is unsuitable for permanent filtration of water under anaerobic conditions.
From xe2x80x9cHandbook of Water Purificationxe2x80x9d, 1981, Walter Lorch, page 307, it is known that ferrous ions in feed water oxidize into ferric ions under the influence of oxygen, which results in precipitation.
Japanese patent publication No. 58 133885 relates to a method for removing dissolved oxygen from liquids. Thus, the liquid is brought into continuous contact with gaseous nitrogen, after which the dissolved oxygen is removed together with gaseous nitrogen in a liquid tank. Then the liquid, from which the oxygen has been removed, is supplied to a membrane filtration unit of the reverse osmosis type.
Japanese patent No. 06 126299 relates to a water treatment plant which is successively provided with an active carbon filter, a de-aeration element, as well as a membrane filtration unit of the reverse osmosis type, whereby the water to be treated, which contains dissolved oxygen, is continuously passed through the de-aeration element.
Japanese patent publication No. 58 011091 relates to a method for de-aerating water, wherein first a flocculation agent and a auxiliary substance are injected into the liquid to be treated, which contains oxygen. Then the flow of water thus pre-treated is passed through a sand filter, after which the obtained filtrate is continuously passed through a de-aeration unit. The water thus obtained, from which the oxygen has been removed, is mixed with a reduction agent, for example a sulfite compound, and subsequently supplied to a membrane unit.
Japanese patent publication No. 61 050654 relates to a method for removing dissolved oxygen from washing water by injecting a deoxidizer into the washing water, and subsequently supplying said washing water to a de-aeration unit for the purpose of removing the dissolved oxygen and the carbon dioxide that remain, after which the washing water thus treated is supplied to a membrane filtration unit of the reverse osmosis type.
The above-discussed Japanese patent publications all relate to the continuous removal of oxygen from an oxygen-containing water flow, after which the water flow from which the oxygen has thus been removed is supplied to a membrane filtration unit. The objective of the present invention, on the other hand, is to clear the membrane filtration unit and the valves, pipes, pumps and the like connected thereto of oxygen, or at least to a considerable extent, during the starting phase, whereby the dissolved oxygen that may be present in the water to be purified is in particular not subjected to an additional treatment, so that the water to be purified comprising the dissolved oxygen that is inherently present therein is carried to a membrane filtration unit, in which anaerobic conditions have been created. After all, in practice mainly ground water will be used, which ground water is already contained in the ground under substantially anaerobic conditions, so that it is in particular desirable to maintain said anaerobic conditions during said membrane filtration rather than remove any remaining dissolved oxygen in advance from the aqueous flow to be purified, as the oxygen content thereof is too low to cause flocculation, which is harmful to the membrane filtration unit. The oxygen from the ambient air will cause flocculation, however, in particular the air which is present in pipes, pumps and in the membrane filtration unit.
Such a method for purifying water, using a membrane filtration unit, is also known from European patent application No. 0 655 418, for example. According to the method disclosed in said publication, the water to be purified is supplied to a reactor in which filter membranes are present. The reactor is furthermore provided with a pipe for discharging floating constituents, and with a pipe for discharging settled constituents. Such constituents are formed mainly by oxidation of metals, which oxidized metals are insoluble in the water. The water to be treated which is present in the reactor is injected with ozone so as to suppress the growth of biological constituents, which constituents have an adverse effect on the membrane filtration process. One drawback of such a method is the fact that contamination of the membranes occurs after some time, in spite of the fact that ozone is injected and that floating and settled constituents are discharged. Said contamination may be caused by the organic substances (macromolecules, biomolecules) and other organic substances (colloids, iron hydroxides, manganese hydroxides, calcium salts and the like) which are present in the water to be treated. In addition to that, also bacteria, algae and the like may cause contamination of the membrane surface.
Such contamination of the membrane surface may be of a reversible or of an irreversible nature. In the case of reversible contamination, the membrane surface can be cleaned by rinsing it with a special solution, for example a soap, acid or lye solution. From practice it is known, however, that frequent, that is, monthly cleaning may reduce the life of the membranes considerably, so that the cost of the plant may increase considerably. In the case of irreversible contamination, it is not possible to clean the membrane surface with the above-mentioned agents. The contamination cannot be removed at all and, depending on the rate of contamination, this may result in a very short life of the membranes, which is generally accompanied with a decrease in the productivity. Thus, the cost of the plant will increase considerably.
One way of preventing or strongly reducing contamination which is frequently used in practice is to subject the water to be purified to a pre-treatment. In the embodiment wherein ground water is purified, such a pre-treatment generally consists of an aeration step and one or more filtration steps. In the embodiment wherein surface water is purified, such a pre-treatment consists of a coagulation step, a flocculation step and a filtration step. In both filtration steps, which are carried out under aerobic conditions, suspended substances and iron and manganese flocks, which flocks are considered as iron and manganese hydroxides, are removed to a considerable degree from the starting material to be treated. Although the iron content and the manganese content can thus be reduced to  less than 0.02 and  less than 0.0005 mg/l respectively, the membranes must still be cleaned two to four times a year on average in order to maintain a reasonable water production per membrane element. One drawback of such a pre-treatment is the fact that additional plants must be built, so that the total cost will increase. Besides, such plants require the possible addition of further chemicals, which chemicals may have an adverse effect on the quality of the eventual drinking water. Moreover, malfunctions do occur with such plants, which malfunctions may disturb the production of the eventual drinking water.
The objective of the present invention is to provide a device and a method for purifying water, in particular ground water, using a membrane filtration unit, which method eliminates the aforementioned drawbacks of the prior art.
It is in particular desirable to develop a method and a device for purifying water wherein the pumped-up ground water can be directly supplied to a membrane filtration unit, without serious contamination of the membrane surface occurring thereby.
Another objective of the present invention is to provide a method and a device for purifying water, wherein the concentrate flow obtained at the membrane filtration unit can be discharged in an advantageous manner.
The method as referred to in the introduction is according to the present invention characterized by the following steps:
1) connecting the membrane filtration unit to a rinsing unit; then
2) passing a rinsing agent from the rinsing unit through the membrane filtration unit so as to create anaerobic conditions in the membrane filtration unit; and
3) passing the water to be purified through the membrane filtration unit, in which anaerobic conditions have thus been created in step 2), wherein the connection between the membrane filtration unit and the rinsing unit is then shut off so that the anaerobic conditions in the membrane filtration unit obtained in step 2) are maintained while the water to be purified is passed through said membrane filtration unit.
Subjecting the ground water to a purification under anaerobic conditions (no oxygen) in a membrane filtration unit will prevent the presence of mainly iron and manganese hydroxides in the form of flocks in the water to be treated, thus preventing contamination or, in the worst case, clogging of the membrane. In addition to that, aerobic bacterial growth is not possible, due to the absence of oxygen. The advantage of purification under anaerobic conditions is the fact that the pre-purification steps as described before, which are frequently used in practice, are no longer required, which will reduce the cost of investment and exploitation. It should be apparent that the term xe2x80x9cmembrane filtration unit in which anaerobic conditions have been createdxe2x80x9d as used therein is understood to mean that anaerobic conditions have been created in all pipes, pumps, valves and the like which are connected to the membrane filtration unit.
Although the term ground water will consistently be used in the following description, it should be apparent that besides ground water, also the processing of surface water, process water, industrial water and any other aqueous flow, whereby anaerobic conditions are desirable, is possible by using the method according to the present invention.
The membrane filtration unit to be used in accordance with the present invention is in particular not critical, reverse osmosis (also known as hyperfiltration), nanofiltration as well as ultrafiltration and microfiltration can be used. It is preferred, however, to use nanofiltration or reverse osmosis, in connection with the removal of particles having a particle size of less than about 10 nm. Thus, dissolved ions and organic components are stopped by means of nanofiltration and hyperfiltration.
The creation of anaerobic conditions in the membrane filtration unit according to step 2) is preferably carried out by passing nitrogen gas through the membrane filtration unit. Other ways of carrying out step 2) comprise the passing of a gas which contains hardly any oxygen, for example a noble gas such as argon, through the membrane filtration unit. From an economic point of view, it is preferred to use nitrogen gas, which is available on a large scale. According to such a method, the oxygen which is present in the pipes, the pumps, the valves and the like and in the membrane filtration unit, in particular air, is replaced by the inert nitrogen gas, so that anaerobic conditions are created, which anaerobic conditions are necessary for effecting permanent purification of the water to be cleaned.
In another preferred embodiment of the method according to the present invention, the creation of anaerobic conditions in the membrane filtration unit is carried out by passing water with a low oxygen content through the membrane filtration unit. Since the water to be purified is passed through the membrane filtration unit in step 3) of the method according to the present invention, the membrane surface is cleared of oxygen and already moistened by the passage of water with a low oxygen content therethrough. In addition, any air bubbles in the membrane surface will have disappeared after water with a low oxygen content has been passed therethrough. It is preferred to prepare water with a low oxygen content by passing nitrogen gas through water. It should be noted, however, that any gas which contains hardly any oxygen is suitable for preparing water with a low oxygen content. In addition to that, also the principle of vacuum de-aeration is suitable for preparing water with a low oxygen content. It is also possible, however, to prepare water with a low oxygen content by adding a deoxidizer such as bisulphite thereto.
In one preferred embodiment of the method according to the present invention, step 2) is carried out in two stages, that is, first the membrane filtration unit is blown through with nitrogen gas, and then water with a low oxygen content is passed through. It should be apparent that care is taken when changing over from nitrogen gas to water with a low oxygen content that no oxygen can come into contact with the membrane filtration unit, which might make it impossible to obtain optimum anaerobic conditions.
In a special embodiment of the method according to the present invention, it is preferred to have step 3) preceded by a step wherein the water to be purified is first subjected to a pre-purification treatment for the purpose of removing the suspended constituents that may be present therein. The presence of suspended constituents may cause clogging of the membrane surface, which will have an adverse effect on the productivity of the eventual product water. Such a pre-purification treatment preferably takes place by using a so-called candle filter, wherein no contact with the outside air is possible.
It should be apparent on the basis of the above-described preferred embodiments that the creation of anaerobic conditions in a membrane filtration unit is important in particular during the starting phase of the water purification process. The moment the water to be treated is passed through the membrane filtration unit, in which anaerobic conditions have already been created in accordance with the present invention, the supply of nitrogen gas and/or water with a low oxygen content to the membrane filtration unit will be stopped. Since the successive steps of pumping up ground water, possibly passing it through a candle filter, and subsequently pumping it to the membrane filtration unit has in accordance with the method of the present invention taken place without any significant contact with the outside air during the starting phase of the purification of water, in particular ground water, the complete process will be maintained under anaerobic conditions without additional measures being taken, for example the passing through of nitrogen gas. In some embodiments it is not inconceivable, however, to admix a small amount of water with a low oxygen content to the aqueous flow to be purified, which embodiments also fall within the scope of the appended claims. Thus, the present invention is essentially different from the methods according to the prior art as described above.
In a special embodiment of the present method it is preferred to place the membrane filtration unit underground, near a water-carrying package. A water-carrying package, as is understood by those skilled in the art, refers to an underground, or subsurface, body of water flowing in a distinct channel, the channel being naturally defined and bounded by particular strata, the latter usually referring to layers of sedimentary rock. Thus, water is passed along the membrane surface underground, under anaerobic conditions, whereby product water (permeate) is produced as a result of a pressure being applied. The concentrate flow being produced is returned via a pipe, still under anaerobic conditions, to the same xe2x80x9cwater-carryingxe2x80x9d package or to a package in which the salt concentrations are higher than in the layer from which the water was obtained. In this latter embodiment, the layer from which the water is obtained is prevented from becoming contaminated with a water flow (i.e., concentrate flow) comprising higher salt concentrations. Special advantages that may be mentioned are:
the saving of space; the entire plant can be realized mainly underground,
the continuous presence of an anaerobic environment, without all kinds of technical measures being required to achieve this,
the concentrate produced remaining anaerobic, thus allowing for it to be injected without any problems into the same water-carrying package, or, when opting for a higher yield from the plant and, consequently, a higher salt concentration in the concentrate flow, to be injected into a water-carrying package comprising comparable salt concentrations (mostly salt water strata), and
the ability to opt for low yields, for example, where the original feed water flow yields material that is 10-20% product water and 80-90% concentrate flow, In such an embodiment, taking into consideration the gigantic volume of a water-carrying package, the concentrate flow could be injected into the same stratum, for example, a few hundred meters xe2x80x9cdownstream.xe2x80x9d
Although European patent application No. 0 297 417 discloses a subsurface filter pipe, it is not known therefrom to create the anaerobic conditions that are important according to the present invention.
The present invention furthermore relates to a device for purifying water, in particular ground water, using a membrane filtration unit which is provided with the necessary valves, supply and discharge pipes and pumps, which device is characterized in that a rinsing unit for creating anaerobic conditions is disposed before the membrane filtration unit, which rinsing unit is connected, via a flushing pipe, to the supply pipe for the water to be purified to the membrane filtration unit, whereby the supply pipexe2x80x94flushing pipe connection can be shut off.
The rinsing unit for creating anaerobic conditions is preferably connected to the supply pipe for the water to be purified to the membrane filtration unit. According to such a construction, no xe2x80x9cdeadxe2x80x9d pipes will be created in the present device, which xe2x80x9cdeadxe2x80x9d pipes contain oxygen, and which may cause biological growth and flocculation of iron and manganese hydroxides, therefore. By placing a three-way valve, for example, in the supply pipe for water to be purified to the membrane filtration unit, it will be easy to change over from step 2) to step 3) of the present invention without any risk of ingress of outside air.
The membrane filtration unit to be used in accordance with the present invention is in particular not critical, reverse osmosis (also known as hyperfiltration), nanofiltration as well as ultrafiltration and microfiltration can be used. It is preferred, however, to use nanofiltration or reverse osmosis, in connection with the removal of particles having a particle size of less than about 10 nm. Thus, dissolved ions and organic components are stopped by means of nanofiltration and hyperfiltration.
In one preferred embodiment of the device according to the present invention, the rinsing unit for creating anaerobic conditions comprises a pipe for supplying nitrogen gas and/or a pipe for supplying water with a low oxygen content, wherein both pipes are connected to the supply pipe for the water to be purified to the membrane filtration unit. According to such an embodiment it is possible in a simple manner to create anaerobic conditions in the membrane filtration unit by first blowing through the supply pipe and the membrane filtration unit with nitrogen gas and subsequently passing water with a low oxygen content through the supply pipe and the membrane filtration unit. It is also possible, however, to blow through nitrogen gas and pass water with a low oxygen content via one pipe, which is connected to the supply pipe for water to be purified to the membrane filtration unit.
If the water to be purified contains suspended constituents which may cause clogging of the membrane surface, it is preferred for the device according to the present invention to comprise a pre-purification unit. Such a pre-purification unit for removing suspended constituents is preferably disposed before the rinsing unit for creating anaerobic conditions. It is preferred to use one or more candle filters as a pre-purification unit. From practice it has become apparent that suspended constituents oxidize rather easily and will cause problems once they have come into contact with oxygen, so that it is recommended to remove such suspended constituents from the water to be treated before the anaerobic conditions are created. It is also possible, however, to rinse such a pre-purification unit, using the rinsing unit.
In a special embodiment of the present invention it is preferred to dispose the membrane filtration unit underground, near a water carrying package, whereby it is furthermore preferred to return the obtained concentrate flow, via a discharge pipe, to the same water carrying package as that in which the membrane filtration unit is disposed. It is also possible, however, to return the concentrate flow, via a discharge pipe, to a water carrying package other than that in which the membrane filtration unit is disposed.